Is bigger better? A comparison of growth restriction rates using the crown-rump length derived due date from the smaller versus larger twin

BACKGROUND: Although the smaller twin's crown-rump length is most accurate in establishing the estimated due date in dichorionic gestations, societal guidelines favor the use of the larger twin measurements based on concern for missing a diagnosis of fetal growth restriction. OBJECTIVE: This study aimed to compare the accuracy of the diagnosis of early- and late-onset fetal growth restriction in dichorionic twin gestations conceived by assisted reproductive technology using the estimated due date as established by the crown-rump length of the smaller vs larger twin. STUDY DESIGN: This was a 10-year retrospective cohort study of nonanomalous, dichorionic gestations conceived with assisted reproductive technology at 2 institutions. The incidence of early-onset (<32 weeks of gestation) and late-onset (≥32 weeks of gestation) growth restriction derived from the Hadlock formula using the smaller and larger crown-rump length estimated due date was compared with the true estimated due date by assisted reproductive technology. Statistical significance was determined using the Fisher exact test. The incidence of missed fetal growth restriction cases, false-positive rate, and error were calculated along with the relative risk for a missed diagnosis using the smaller crown-rump length. RESULTS: A total of 176 subjects were screened: 81 had a fetal growth ultrasound at 24 to <32 weeks of gestation, and 58 had a fetal growth ultrasound at ≥32 weeks of gestation. There was a significant difference in the incidence of fetal growth restriction using the 3 dating strategies in both gestational age ranges (P<.001) with the smaller crown-rump length estimated due date more closely approximating the true rate. Before 32 weeks of gestation, the smaller crown-rump length estimated due date missed 2.5% of fetal growth restriction cases, whereas the larger crown-rump length estimated due date missed 0.6% of fetal growth restriction cases, with false-positive and error rates of 1.2% and 3.7% and 5.5% and 6.2%, respectively. After 32 weeks of gestation, the smaller crown-rump length estimated due date missed 1.8% of cases, whereas the larger crown-rump length estimated due date missed 0% of cases, with false-positive and error rates of 2.6% and 4.4% and 5.3% and 5.3%, respectively. The relative risk for a missed diagnosis of fetal growth restriction using the smaller crown-rump length estimated due date was 1.77 for early-onset growth restriction and 1.22 for late-onset growth restriction. CONCLUSION: Using the estimated due date derived from the smaller twin led to a more accurate detection of fetal growth restriction at a cost of a higher missed diagnosis rate.